Wind turbine having eigenfrequency modifier

ABSTRACT

It is provided a wind turbine having a modified eigenfrequency, the wind turbine having a tower including a first tower flange arranged at an upper end portion of a top part of the tower; and, an eigenfrequency modifier including a plurality of weights suspended from the first tower flange by a plurality of rigid supports. It is further provided a method of modifying an eigenfrequency of a wind turbine, the method including modifying the eigenfrequency of the wind turbine by suspending a plurality of weights from a first tower flange arranged at an upper end portion of a top part of a tower of the wind turbine using a plurality of rigid supports.

FIELD

The present disclosure relates generally to wind turbines, and inparticular to a wind turbine having an eigenfrequency modifier and amethod of modifying an eigenfrequency of a wind turbine.

BACKGROUND

Wind power is considered one of the cleanest, most environmentallyfriendly energy sources presently available, and wind turbines havegained increased attention in this regard. A modern wind turbinetypically includes a tower, generator, gearbox, nacelle, and one or morerotor blades. The rotor blades capture kinetic energy of wind usingknown airfoil principles. The rotor blades transmit the kinetic energyin the form of rotational energy so as to turn a shaft coupling therotor blades to a gearbox, or if a gearbox is not used, directly to thegenerator. The generator then converts the mechanical energy toelectrical energy that may be deployed to a utility grid.

Important fundamental vibrations of the wind turbine include naturalbending modes of the tower and rotor components. The tower, nacelle, androtor vibration modes may also interact or couple under certainconditions and cause an operational instability. Harmonic rotor loadsthat occur at multiples of the rotor speed can also excite the system'snatural vibration modes. A fundamental system frequency that is drivenby a harmonic rotor load is called a resonance condition. Potentiallylarge structural displacements can result from a resonance condition.Thus, there is a problem of resonance condition in wind turbines.

BRIEF DESCRIPTION

In view of the above, a wind turbine having an eigenfrequency modifierand a method of modifying an eigenfrequency of a wind turbine isdisclosed. Aspects and advantages will be set forth in part in thefollowing description.

In one aspect, the present disclosure is directed to a wind turbinehaving a modified eigenfrequency, the wind turbine having a towerincluding a first tower flange arranged at an upper end portion of a toppart of the tower; and, an eigenfrequency modifier including a pluralityof weights suspended from the first tower flange by a plurality of rigidsupports.

In another aspect, the present disclosure is directed to a method ofmodifying an eigenfrequency of a wind turbine, the method includingmodifying the eigenfrequency of the wind turbine by suspending aplurality of weights from a first tower flange arranged at an upper endportion of a top part of a tower of the wind turbine using a pluralityof rigid supports.

These and other features, aspects and advantages of the presentdisclosure will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustratethe present disclosure and, together with the description, serve toexplain the principles of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 is a perspective view of a wind turbine according to anembodiment of the present disclosure;

FIG. 2 is a simplified internal view of a nacelle of a wind turbineaccording to an embodiment of the present disclosure;

FIG. 3 is a simplified illustration of a wind turbine having aneigenfrequency modifier according to an embodiment of the presentdisclosure;

FIG. 4 is a simplified illustration of a weight of an eigenfrequencymodifier according to an embodiment of the present disclosure;

FIG. 5 is a simplified illustration of eigenfrequency modifier accordingto an embodiment of the present disclosure;

FIG. 6 is a cut-out view of a top part of a wind turbine tower accordingto an embodiment of the present disclosure;

FIG. 7 is an internal view of top part of a wind turbine tower accordingto an embodiment of the present disclosure;

FIG. 8 is a schematic illustration of a method of modifying aneigenfrequency of a wind turbine according to an embodiment of thepresent disclosure; and

FIG. 9 is a schematic illustration of a method of modifying aneigenfrequency of a wind turbine according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the various embodiments, one ormore examples of which are illustrated in each figure. Each example isprovided by way of explanation and is not meant as a limitation. Forexample, features illustrated or described as part of one embodiment canbe used on or in conjunction with any other embodiment to yield yet afurther embodiment. It is intended that the present disclosure includessuch modifications and variations.

Within the following description of the drawings, the same referencenumbers refer to the same or to similar components. Generally, only thedifferences with respect to the individual embodiments are described.Unless specified otherwise, the description of a part or aspect in oneembodiment applies to a corresponding part or aspect in anotherembodiment as well.

Referring now to the drawings, FIG. 1 shows a perspective view of a windturbine according to the present disclosure. As shown, the wind turbine10 generally includes a tower 12 extending from a support surface 14, anacelle 16 mounted on the tower 12, and a rotor 18 coupled to thenacelle 16.

As shown in FIG. 1, the rotor 18 includes a rotatable hub 20 and atleast one rotor blade 22 coupled to and extending outwardly from the hub20. For example, in the illustrated embodiment, the rotor 18 includesthree rotor blades 22. However, in an alternative embodiment, the rotor18 may include more or less than three rotor blades 22. Each rotor blade22 may be spaced about the hub 20 to facilitate rotating the rotor 18 toenable kinetic energy to be transferred from the wind into usablemechanical energy, and subsequently, electrical energy. For instance,the hub 20 may be rotatably coupled to an electric generator 24 (FIG. 2)positioned within the nacelle 16 to permit electrical energy to beproduced.

The wind turbine 10 may also include a wind turbine controller 26centralized within the nacelle 16. However, in other embodiments, thecontroller 26 may be located within any other component of the windturbine 10 or at a location outside the wind turbine 10. Further, thecontroller 26 may be communicatively coupled to any number of thecomponents of the wind turbine 10 in order to control the components. Assuch, the controller 26 may include a computer or other suitableprocessing unit. Thus, in several embodiments, the controller 26 mayinclude suitable computer-readable instructions that, when implemented,configure the controller 26 to perform various different functions, suchas receiving, transmitting and/or executing wind turbine controlsignals.

Referring now to FIG. 2, a simplified internal view of a nacelle 16 of awind turbine 10, particularly illustrating the drivetrain componentsthereof, is shown. More specifically, as shown, the generator 24 may becoupled to the rotor 18 for producing electrical power from therotational energy generated by the rotor 18. The rotor 18 may be coupledto the main shaft 34, which is rotatable via a main bearing (not shown).The main shaft 34 may, in turn, be rotatably coupled to a gearbox outputshaft 36 of the generator 24 through a gearbox 30.

The gearbox 30 may include a gearbox housing 38 that is connected to thebedplate 46 by one or more torque arms 48. More specifically, in certainembodiments, the bedplate 46 may be a forged component in which the mainbearing (not shown) is seated and through which the main shaft 34extends. As is generally understood, the main shaft 34 provides a lowspeed, high torque input to the gearbox 30 in response to rotation ofthe rotor blades 22 and the hub 20. Thus, the gearbox 30 thus convertsthe low speed, high torque input to a high speed, low torque output todrive the gearbox output shaft 36 and thus, the generator 24.

Each rotor blade 22 may also include a pitch adjustment mechanism 32configured to rotate each rotor blade 22 about its pitch axis 28 via apitch bearing 40. Similarly, the wind turbine 10 may include one or moreyaw drive mechanisms 42 communicatively coupled to the controller 26,with each yaw drive mechanism(s) 42 being configured to change the angleof the nacelle 16 relative to the wind (e.g., by engaging a yaw bearing44 of the wind turbine 10). The yaw bearing 44 may be arranged on and/orsupported by a first tower flange 530, e.g. arranged on and/or supportedby a flange arranged at an upper end portion of a top part 510 of thetower 12.

As described, a resonance condition in wind turbines is undesirable. Forexample, a wind turbine having an excitation frequency (e.g. a rotorfrequency, also referred to as 1P frequency; or a blade passingfrequency, also referred to as 3P frequency) substantially coincidentwith a natural frequency of the wind turbine 10 (e.g. a first naturalfrequency or first eigenfrequency of the wind turbine) is undesirable.In particular, a wind turbine 10 having a first eigenfrequency notsufficiently separated from the rotor frequency of the wind turbine 10is undesirable.

For example, when there is insufficient separation of the firsteigenfrequency and the rotor frequency, the risk of turbine heavilyoscillating may become unacceptable. In such a case, long-term fatiguedamage, risk of immediate damage to the equipment, and to environment,e.g. in case of catastrophic failure, constitute undesirableconsequences.

A wind turbine with an excitation frequency insufficiently separatedfrom a natural frequency may occur despite the wind turbine beingdesigned to have sufficient separation. For example, deviations mayoccur in top mass, e.g., due to vendor tolerances in components, and infoundation stiffness, e.g., due to site conditions.

In particular, deviations from design specifications may occur duringmanufacturing, e.g., due to vendor tolerances, during construction,e.g., because of variation in foundation stiffness, and even duringoperation during the wind turbine's lifetime, e.g., due to componentreplacements, weathering effects on foundation stiffness etc.

The embodiments described herein addresses the problem of insufficientseparation between an excitation frequency and a natural frequency of awind turbine 10 in an optimal manner. For example, a wind turbine 10having an eigenfrequency modifier 350 including a plurality of weights450 suspended, by a plurality of rigid supports 550, from a first towerflange 530 arranged at an upper end portion of a top part 510 of thetower 12 can be installed.

Accordingly, deviations arising from a variety of causes (vendortolerances, foundation stiffness), at a variety of times (duringmanufacturing, construction, operation etc.), can be compensated and aseparation of the natural frequency and an excitation frequency can beincreased.

In particular, the weights being suspended from the first tower flange530 is particularly effective because of the arrangement near the top ofthe tower 12. For example, the first tower flange 530 may be the flangeon which the yaw bearing 44 is arranged. In a further example, the firsttower flange 530 may be directly below the yaw bearing 44. Accordingly,the first tower flange 530 is in a relatively optimal position foraffecting the natural frequency.

It may be understood that space is extremely limited within the nacelleand tower 12. Thus, an arrangement of suspending from the first towerflange 530 is particularly beneficial for minimizing interference withother equipment and access. Further, an arrangement of suspending fromthe first tower flange 530 is particularly beneficial because existingdesigns can easily incorporate a plurality of rigid supports 550suspending weights from the first tower flange 530 with minimal, if any,modifications to the design.

According to an aspect, there is provided a wind turbine 10 having amodified eigenfrequency including a tower 12 including a first towerflange 530 arranged at an upper end portion of a top part 510 of thetower 12; and, an eigenfrequency modifier 350 including a plurality ofweights 450 suspended from the first tower flange 530 by a plurality ofrigid supports 550.

According to an embodiment, the plurality of rigid supports 550 includesat least a first rigid support and a second rigid support, the secondrigid support arranged at a first circumferential distance apart fromthe first rigid support. According to an embodiment, each of theplurality of weights 450 is supported by at least two rigid supports ofthe plurality of rigid supports 550.

For example, the first rigid support and the second rigid support may bearranged at substantially the same radius from the axis of the tower 12.In an example, each of the plurality of weights 450 are annular sectorshaped.

In particular, the first rigid support may include a first bar anchoredto the first tower flange 530 at a first radius from the axis of thetower and at a first angular position. In particular, the second rigidsupport may include a second bar anchored to the first tower flange 530at a second radius from the axis of the tower 12 and at a second angularposition.

In particular, the first radius and the second radius may besubstantially the same. In particular, the first angular position andthe second angular position may be different. In particular, anembodiment formed by the combination of the described examples has thebenefits described herein.

For example, the first angular position and the second angular positionare at least 15 degrees apart or at least 30 degrees apart. For example,the first angular position and the second angular position are less than90 degrees apart and/or less than 60 degrees apart. In particular, eachof the plurality of weights 450 may have a width (in the circumferentialdirection) at least as wide as the spacing (in the circumferentialdirection) of the first rigid support and second rigid support.

A circumferential direction may be understood as in a direction parallelto a wall of the tower 12. Accordingly, large angle annular sectorshaped weights can be provided, each being supported at, at least two,circumferentially spaced apart support points. Beneficially, such amounting configuration allows limited tower space to be usedefficiently.

According to an embodiment, each of the plurality of weights 450 has awidth dimension or outer arc length dimension of at least 1%, at least2.5%, or at least 5% of a circumference dimension of the tower 12. Forexample, each of the plurality of weights 450 is of an annular sectorshape with an angular dimension of at least 3.6 degrees, at least 9degrees, or at least 18 degrees. Beneficially, when an angular extent ofeach of the plurality of weights 450 is larger, less rigid supports andless support points are needed, and mounting requirements are eased.

According to an embodiment, each of the plurality of weights 450 has awidth dimension or outer arc length dimension of at most 15%, at most10%, or at most 7.5% of a circumference dimension of the tower 12. Forexample, each of the plurality of weights 450 is of an annular sectorshape with an angular dimension of at most 54 degrees, at most 36degrees or at most 27 degrees. Beneficially, when an angular extent ofeach of the plurality of weights 450 is smaller, handling requirementsare eased, particularly during lifting and installation.

It is explicitly indicated that a plurality of combinations of numericalranges are disclosed and provided herein, but not listed explicitly forsake of clarity. Each particular combination of numerical ranges providecorresponding effects. For example, each of the plurality of weights 450may be of an annular sector shape with an angular dimension of at least3.6 degrees, and at most 36 degrees. Beneficially, a correspondingbalance of mounting requirements and handling requirements is provided.

In a particular example, the circumference dimension of the tower 12 maybe a circumference dimension of the upper end portion of the top part510 of the tower 12. A width dimension or outer arc length dimension ofeach of the plurality of weights 450 may be understood as a dimensionsubstantially parallel to a tangent of the circumference of the tower 12or to the circumference of the tower 12. Beneficially, limited space inthe tower is used efficiently.

According to an embodiment, each of the plurality of weights 450 arearranged at most 100 mm, at most 50 mm, or at most 25 mm from an insidesurface of the tower 12. For example, each of the plurality of weights450 is arranged immediately adjacent to an inside surface of the tower12. In particular, the inside surface of the tower 12 may be understoodas an inside surface of the upper end portion of the top part 510 of thetower 12. Beneficially, limited space in the tower is used efficiently.

According to an embodiment, each of the plurality of weights 450 have adepth dimension of at most 1000 mm, at most 500 mm, or at most 250 mm.For example, each of the plurality of weights 450 has a depth dimensionof at most 20% of a diameter of the tower 12, at most 10% of a diameterof the tower 12, or at most 5% of a diameter of the tower 12.

A depth dimension may be understood as a dimension substantiallyparallel to or collinear with the radius or diameter of the tower 12. Adiameter of the tower 12 may be understood as a diameter of the upperend portion of the top part 510 of the tower 12.

Beneficially, limited space in the tower is used efficiently.

According to an embodiment, each of the plurality of weights 450 has awidth dimension or outer arc length dimension to depth dimension of atleast 2 to 1, at least 5 to 1, or at least 10 to 1. Beneficially,limited space in the tower is used efficiently.

According to an embodiment, each of the plurality of weights 450 has amass of at most 200 kg, at most 100 kg or at most 50 kg. In an example,each of the plurality of weights 450 can be carried up the tower 12 byelevator. In an example, the elevator is inside the tower 12.Beneficially, handling is eased. Beneficially, the plurality of weights450 can be better handled, particularly in the limited space within thetower.

In an example, the plurality of weights 450 are installed beforeinstallation of the nacelle, when the final mass of the nacelle androtor assembly as installed is known. In another example, the pluralityof weights 450 are installed after the installation of the nacelle, whenthe eigenfrequency of the wind turbine as installed is known or ismeasured. The eigenfrequency of the wind turbine may be measured afterthe installation or assembly of the entire wind turbine.

In an example, the plurality of weights 450 are lifted with a hoistinside the tower. In an example, the hoist may be an internal hoist. Inan example, the plurality of weights 450 may be lifted one by one.

According to an embodiment, the plurality of weights 450 includes atleast a first weight and a second weight arranged on top of the firstweight. According to an embodiment, the first weight and second weightare suspended from the first tower flange 530 by the same rigid supportor rigid supports.

For example, the first weight and the second weight may substantiallyoverlap in a plane perpendicular to the axis of the tower 12.

In particular, each rigid support may include a bar extending in adirection parallel to gravity. In particular, the first weight andsecond weight may be arranged on top of each other along a bar of arespective rigid support along an axis parallel to gravity.

Accordingly, a stack of at least two weights can be supported by eachmount. Beneficially, such a mounting configuration allows limitedmounting points on the first tower flange 530 to be used efficiently.

According to an embodiment, dimensions and material of the plurality ofweights 450 are standardized. For example, the plurality of weights 450may have dimensions that are uniform between each weight or may have oneof a pre-determined set of dimensions. For example, each of theplurality of weights 450 is of the same material.

In particular, each of the plurality of weights 450 may have a firstside with the same first radius of curvature and/or a second side withthe same second radius of curvature. In particular, the first radius ofcurvature may be less than the second radius of curvature, and/or thesecond radius of curvature may be less than a radius of curvature of aninterior surface of an upper end portion of the top part 510 the tower12.

In particular, each of the plurality of weights 450 may have a thirdside and a fourth side, wherein the third side and fourth side are ofthe same dimensions. In particular, each of the plurality of weights 450may have a first thickness, wherein the first thickness of each weightis one of a pre-determined set of thickness. In particular, each of theplurality of weights 450 may be of a metal, more particularly, of steel.

Accordingly, the numerous benefits afforded by pre-fabrication,universal compatibility, and mass-production are gained. Beneficially,eigenfrequency modifiers can be configured for a specific wind turbineextremely efficiently, at any time, e.g. during commissioning or duringoperation of the wind turbine 10.

Beneficially, eigenfrequency modifiers can be specifically configuredfor each individual wind turbine of a plurality of wind turbinesextremely efficiently, at any time, e.g. during commissioning or duringoperation of each individual wind turbine of the plurality of windturbines.

According to an embodiment, each of the plurality of rigid supports 550includes a bar. According to an embodiment, each of the plurality ofweights 450 includes a through-slot 470 adapted for receiving the bar.According to an embodiment, the through-slot 470 includes an open endadapted for receiving the bar.

For example, each of the plurality of rigid supports 550 includes a barhaving a first diameter. For example, each of the plurality of weights450 includes a through-slot 470 having a second diameter substantiallyequal to the first diameter.

In particular, the through-slot 470 may be open-ended on one side of therespective weight, thus allowing the bar to be accommodated in thethrough-slot 470 of the weight via the open-end of the through-slot 470.

Accordingly, each weight can be slotted onto the bar. Beneficially, theinstallation of many heavy weights is made straightforward, particularlyin the limited space available inside the tower 12, and with minimal, ifany, modifications to the design of the tower 12.

According to an embodiment, the plurality of weights 450 are suspendedat least a first distance from the first tower flange 530. According toan embodiment, the first distance is sufficient for a torque tool tooperate on the first tower flange 530. A torque tool may be a spannerand/or wrench.

For example, the first distance is sufficient for a torque tool tooperate on a bolt connection 590 of the first tower flange 530. Inparticular, the bolt connection 590 may be a bolt connection 590connecting the yaw bearing 44 to the first tower flange 530. The firstdistance may be understood as along a direction perpendicular to abottom surface of the flange.

Accordingly, the first tower flange 530 is used as a mounting point forthe plurality of weights 450 while maintaining the necessary access tothe first tower flange 530. Beneficially, the limited space around thefirst tower flange 530 having existing access demands can be usedwithout compromising access to flange, thus avoiding modifications tothe design of the tower 12.

According to an embodiment, the eigenfrequency modifier 350 includes atleast a first magnetic coupler 570 coupling a bottom end portion of eachof the plurality of rigid supports 550 to the tower 12. Accordingly,vibration of the eigenfrequency modifier 350 is reduced and/or damped ina simple manner.

For example, vibration is damped without introducing complex couplingand tolerance considerations. In particular, the first magnetic coupler570 may be fully compatible with any configuration, e.g. differentamount of weights, required of the eigenfrequency modifier 350.Beneficially, vibration of an adaptable eigenfrequency modifier 350 isavoided, reduced and/or damped in a particularly simple manner.

According to an embodiment, the eigenfrequency modifier 350 includes aplurality of lateral supports 770. According to an embodiment, each ofthe plurality of lateral supports 770 protrudes from an interior side ofthe tower 12. According to an embodiment, each of the plurality oflateral supports 770 is arranged at a height position at least partiallyoverlapping a height position of the plurality of weights 450.

For example, each of the plurality of lateral supports 770 is arrangedadjacent to a lateral side of the plurality of weights 450. Inparticular, each of the plurality of lateral supports 770 may be a bossattached or welded to an interior side of the tower 12.

According to embodiments, each of the plurality of lateral supports 770is arranged adjacent to the plurality of weights 450. In particular,each of the plurality of lateral supports 770 may be configured toconstrain a lateral (and/or inward radial) movement of the plurality ofweights 450.

In particular, each of the plurality of lateral supports 770 may beinstalled after the plurality of weights 450 are installed. Inparticular, each of the plurality of lateral supports 770 may bepermanently attached to the tower 12, e.g. welded. In particular, eachof the plurality of lateral supports 770 is configured for securing theplurality of weights 450 in an installed position.

Accordingly, lateral movement of the plurality of weights 450 suspendedat a distance from the first tower flange 530 may be sufficientlyconstrained with minimal, if any, modifications to the design of thetower 12. Beneficially, a reliability of the suspension of weights at adistance from first tower flange 530 is ensured in a simple manner.

According to an embodiment, a density of each of the plurality ofweights 450 is more than 2000 kg/m3. For example, a material of each ofthe plurality of weights 450 is concrete. According to an embodiment, adensity of each of the plurality of weights 450 is more than 6000 kg/m3.For example, a material of each of the plurality of weights 450 is castiron or a steel alloy.

Accordingly, the density as described achieves, in a particularlyeffective manner, a requisite separation of excitation frequency andnatural frequency of the wind turbine. In particular, the densityminimum as described is particularly significant considering thenecessary masses, space limitation, and existing access considerationsinvolved.

In an example, without particular limitation, for illustrative purpose,a top mass of a wind turbine 10 can be in the range of 250 to 300 tons.In the example, a large amount of weight is thus needed. For example, amass of the weights may be of the order of a ton or of the order of 10tons. For example, the weights may be at least 2 tons, at least 5 tons,or at least 10 tons.

In the example, the frequency separation between the rotor frequency andfirst eigenfrequency may be thus increased by about 1%. For example, thefrequency separation may thus be increased from about 5% differencebetween the rotor frequency and first eigenfrequency to about 6%difference between the rotor frequency and first eigenfrequency.

Beneficially, weights having density minimum as described in examplesherein allows limited space to be used efficiently.

According to an embodiment, a specified first eigenfrequency of the windturbine 10 as designed is less than a rotor frequency or a blade passingfrequency of the wind turbine. For example, a tower design of the windturbine 10 is considered soft-soft.

It is understood that a tower design of a wind turbine 10 may bereferred to as ‘soft-soft’ or ‘soft-stiff’ depending on the firsteigenfrequency of the wind turbine 10 (or a natural frequency of a firstmode of vibration of the wind turbine).

It is understood that a wind turbine of a ‘soft-soft’ tower design has afirst eigenfrequency lower than a rotor frequency. It is understood thata wind turbine of a ‘soft-stiff’ tower design has a first eigenfrequencyhigher than a rotor frequency but lower than a blade passing frequency.

It can be understood that a rotor frequency is a frequency of the rotorof the wind turbine when operating at nominal speed.

It can be understood that a wind turbine having a ‘soft-soft’ towerdesign has a first eigenfrequency that coincides with an excitation ofthe rotor at some point between cut-in or start-up of the wind turbinebut not when the wind turbine is operating at its rated RPM or nominalspeed.

An eigenfrequency described herein can be understood as a firsteigenfrequency or a first natural frequency of the wind turbine 10.Accordingly, the plurality of weights lowers the first eigenfrequency,thereby increasing the frequency separation of the first eigenfrequencyand an excitation frequency. Beneficially, a frequency separation of anexcitation frequency and natural frequency is increased.

According to an aspect, there is provided a method of modifying aneigenfrequency of a wind turbine comprising modifying the eigenfrequencyof the wind turbine by suspending a plurality of weights from a firsttower flange arranged at an upper end portion of a top part of a towerof the wind turbine using a plurality of rigid supports 860.

According to an embodiment, the method includes determining a mass ofthe plurality of weights during commissioning or operation of the windturbine 940. Accordingly, deviations arising at a variety of times afterthe design phase, e.g., during manufacturing, construction, operationetc., is rectified. Beneficially, the problem of insufficient separationbetween an excitation frequency and a natural frequency of a windturbine is addressed in an optimal manner, and an undesirably largesafety margin or design tolerance in eigenfrequency separation can beavoided.

According to an embodiment, the method includes determining, based on adifference between a specified eigenfrequency of the wind turbine asdesigned and an eigenfrequency of the wind turbine as installed, anamount of the plurality of weights to be suspended 950. Accordingly,deviations arising from a variety of causes (vendor tolerances,foundation stiffness), is rectified.

Beneficially, the problem of insufficient separation between anexcitation frequency and a natural frequency of a wind turbine isaddressed in an optimal manner, and an undesirably large safety marginor design tolerance in eigenfrequency separation can be avoided.

According to an embodiment, the method includes, prior to suspending theplurality of weights, positively determining at least one of thefollowing: (i) a tower top mass of the wind turbine is less than aspecified lower limit of the tower top mass of the wind turbine 910;(ii) a stiffness of a foundation of the wind turbine is more than aspecified upper limit of the stiffness of the foundation of the windturbine 920; and, (iii) an eigenfrequency of the wind turbine is morethan a specified upper limit of the eigenfrequency of the wind turbine930.

The tower top mass may be understood as a top mass of the wind turbine.

In particular, the tower top mass of the wind turbine may be the actualtower top mass of the wind turbine. In an example, the tower top mass ofthe wind turbine is the tower top mass of the wind turbine as installed.In a further example, the tower top mass of the wind turbine is thein-field tower top mass of the wind turbine.

In an example, the tower top mass of the wind turbine is determinedbased on post-fabrication measurements. In a further example, the towertop mass of the wind turbine is determined based on in-fieldmeasurements.

The specified lower limit of the tower top mass of the wind turbine maybe understood as a value lower than the tower top mass as designed. Inan example, the specified lower limit of the tower top mass of the windturbine may be a designed lower limit of the tower top mass.

In an example, the specified lower limit of the tower top mass of thewind turbine is a value that is 80%, 90% or 95% of the designed towertop mass.

In an example, the specified lower limit of the tower top mass of thewind turbine is a value that is 30 tons, 20 tons or 10 tons less thanthe designed tower top mass.

In particular, the stiffness of the foundation of the wind turbine maybe the actual stiffness of the wind turbine. In an example, thestiffness of the foundation of the wind turbine is the stiffness of thefoundation of the wind turbine as installed. In another example, thestiffness of the foundation of the wind turbine is the in-fieldstiffness of the foundation of the wind turbine.

In an example, the stiffness of the foundation of the wind turbine isdetermined based on post-construction measurements. In a furtherexample, the stiffness of the foundation of the wind turbine isdetermined based on in-field measurements.

The specified upper limit of the stiffness of the foundation of the windturbine may be understood as a value higher than the stiffness of thefoundation as designed. In an example, the specified upper limit of thestiffness of the foundation of the wind turbine may be a designed upperlimit of the stiffness of the foundation.

In an example, the specified upper limit of the stiffness of thefoundation of the wind turbine is a value that is 10 times, 100 times or1000 times of the designed stiffness of the foundation.

In an example, the specified upper limit of the stiffness of thefoundation of the wind turbine is a value that is 10{circumflex over( )}8 kNm/rad, 10{circumflex over ( )}9 kNm/rad or 10{circumflex over( )}10 kNm/rad more than the designed stiffness of the foundation.

In a further example, the specified upper limit of the stiffness of thefoundation of the wind turbine is a value that is 10{circumflex over( )}3 kN/mm, 10{circumflex over ( )}4 kN/mm or 10{circumflex over ( )}5kN/mm more than the designed stiffness of the foundation.

The eigenfrequency of the wind turbine may be understood as the firsteigenfrequency of the wind turbine or as the natural frequency of thewind turbine.

In particular, the eigenfrequency of the wind turbine may be the actualeigenfrequency of the wind turbine. In an example, the eigenfrequency ofthe wind turbine is the eigenfrequency of the wind turbine as installed.In another example, the eigenfrequency ss of the wind turbine is thein-field eigenfrequency of the wind turbine.

In an example, the eigenfrequency of the wind turbine is determinedbased on post-assembly measurements or post-installation measurements.In a further example, the eigenfrequency of the wind turbine isdetermined based on in-field measurements.

The specified upper limit of the eigenfrequency of the wind turbine maybe understood as a value higher than the eigenfrequency of the windturbine as designed. In an example, the specified upper limit of theeigenfrequency of the wind turbine may be a designed upper limit of theeigenfrequency of the wind turbine.

In an example, the specified upper limit of the eigenfrequency of thewind turbine is a value that is 120%, 110% or 105% of the designedeigenfrequency.

In an example, the specified upper limit of the eigenfrequency of thewind turbine is a value that is 0.1 Hz, 0.01 Hz or 0.005 Hz more thanthe designed eigenfrequency.

The specified upper limit of the eigenfrequency of the wind turbine maybe understood as a value that is equal to or less than the rotorfrequency of the wind turbine.

In an example, the specified upper limit of the eigenfrequency of thewind turbine is a value that is 80%, 90% or 95% of the rotor frequencyof the wind turbine.

In an example, the specified upper limit of the eigenfrequency of thewind turbine is a value that is 0.1 Hz, 0.01 Hz or 0.005 Hz less thanthe rotor frequency of the wind turbine.

Accordingly, the optimal conditions for applying the eigenfrequencymodification is identified. Beneficially, the eigenfrequencymodification is effectively applied.

According to an aspect, there is provided a method of customizing aneigenfrequency of each of a plurality of wind turbines in-field,including for each of the plurality of wind turbines 970, performing themethod according to aspects or embodiments described herein.

Accordingly, an eigenfrequency of each of a plurality of wind turbinesindividually adapted according to a standardized method. Beneficially,deviations arising from a variety of causes and at a variety of timescan be efficiently addressed.

Referring now to FIG. 3, a schematic illustration of a wind turbine 10having an eigenfrequency modifier 350 is shown. The eigenfrequencymodifier 350 is illustrated within the tower 12.

Referring now to FIG. 4, one of a plurality of weights 450 is shown. Theweight is illustrated to include a through-slot 470.

Referring now to FIG. 5, a simplified illustration of an eigenfrequencymodifier 350 is shown. The eigenfrequency modifier 350 is illustrated toinclude a plurality of weights 450 suspended from the first tower flange530 by a plurality of rigid supports 550. As illustrated, theeigenfrequency modifier 350 includes a first magnetic coupler 570. Asillustrated, the first tower flange 530 is arranged at an upper endportion of a top part 510, and includes bolt connection 590.

Referring now to FIG. 6, a cut-out view of a top part 510 of a windturbine tower is shown. A eigenfrequency modifier space 610 for theeigenfrequency modifier 350 is illustrated. The eigenfrequency modifierspace 610 is illustrated below the first tower flange 530 and within atop part 510 of the tower 12.

The eigenfrequency modifier space 610 may be understood as a space whereat least some (or all) of the plurality of weights 450 of theeigenfrequency modifier 350 may be arranged. It may be understood thateach of the plurality of weights 450 has an angular dimension or angularextent of less than 360 degrees, for example, at most 54 degrees, atmost 36 degrees or at most 27 degrees.

Referring now to FIG. 7, an internal view of top part 510 of a windturbine tower 12 is shown. Illustrated is the eigenfrequency modifier350 including a plurality of weights 450 suspended from the first towerflange 530 by a plurality of rigid supports 550. Further illustrated isthe plurality of weights 450 being secured by a plurality of lateralsupports 770.

Referring now to FIG. 8, a method of modifying an eigenfrequency of awind turbine 860 is schematically illustrated. The method is illustratedto include suspending the weights from the first tower flange at theupper end portion of the top part of the tower 860.

Referring now to FIG. 9, a method of modifying an eigenfrequency of awind turbine 910-970 is schematically illustrated. The method isillustrated to include at least one of, determining the top mass is lessthan a lower limit 910, determining the foundation stiffness is morethan an upper limit 920, determining the eigenfrequency is more than therotor frequency lower limit 930, determining the amount of weightsduring commissioning or operation of the wind turbine 940, determiningthe weights based on a difference between the as-designed andas-installed eigenfrequency 950, suspending the weights from the firsttower flange at the upper end portion of the top part of the tower 860,and customizing an eigenfrequency of each of a plurality of windturbines 970.

This written description uses examples to describe the presentdisclosure, including the best mode, and also to enable any personskilled in the art to practice the present disclosure, including makingand using any devices or systems and performing any incorporatedmethods. The scope of the invention is defined by the claims.

1-15. (canceled)
 16. A wind turbine, comprising: a tower; a first towerflange arranged at an upper end portion of a top part of the tower; aneigenfrequency modifier; and wherein the eigenfrequency modifiercomprises a plurality of weights suspended from the first tower flangeby a plurality of rigid supports.
 17. The wind turbine according toclaim 16, wherein the plurality of rigid supports comprises a firstrigid support and a second rigid support, the second rigid supportarranged at a first circumferential distance apart from the first rigidsupport; and, wherein each of the plurality of weights is supported byat least two of the rigid supports.
 18. The wind turbine according toclaim 16, wherein the plurality of weights includes a first weight and asecond weight arranged on top of the first weight; and wherein the firstweight and second weight are suspended from the first tower flange by asame one or ones of the rigid supports.
 19. The wind turbine accordingto claim 16, wherein the plurality of weights have standardizeddimensions and material.
 20. The wind turbine according to claim 16,wherein: each of the plurality of rigid supports includes a bar; each ofthe plurality of weights includes a through-slot adapted for receivingthe bar; and the through-slot includes an open end adapted for receivingthe bar.
 21. The wind turbine according to claim 16, wherein theplurality of weights are suspended at a first distance from the firsttower flange that is sufficient for a torque tool to operate on thefirst tower flange.
 22. The wind turbine according to claim 16, whereinthe eigenfrequency modifier comprises a first magnetic coupler couplinga bottom end portion of each of the plurality of rigid supports to thetower.
 23. The wind turbine according to claim 16, wherein: theeigenfrequency modifier comprises a plurality of lateral supports thatprotrude from an interior side of the tower; the lateral supportsarranged at a height position at least partially overlapping a heightposition of the plurality of weights; and each of the plurality oflateral supports arranged adjacent to the plurality of weights.
 24. Thewind turbine according to claim 16, wherein a density of each of theplurality of weights is more than 2000 kg/m3.
 25. The wind turbineaccording to claim 16, wherein a specified first eigenfrequency of thewind turbine is less than a rotor frequency or a blade passing frequencyof the wind turbine.
 26. A method of modifying an eigenfrequency of awind turbine, the method comprising: suspending a plurality of weightsfrom a first tower flange arranged at an upper end portion of a top partof a tower of the wind turbine using a plurality of rigid supports. 27.The method according to claim 26, further comprising determining anamount of the plurality of weights during commissioning or operation ofthe wind turbine.
 28. The method according to claim 26, furthercomprising determining, based on a difference between a specifiedeigenfrequency of the wind turbine as designed and an eigenfrequency ofthe wind turbine as installed, an amount of the plurality of weights tobe suspended.
 29. The method according to claim 26, further comprising,prior to suspending the plurality of weights, determining at least oneof the following: a tower top mass of the wind turbine is less than aspecified lower limit of the tower top mass of the wind turbine; astiffness of a foundation of the wind turbine is more than a specifiedupper limit of the stiffness of the foundation of the wind turbine; and,an eigenfrequency of the wind turbine is more than a specified upperlimit of the eigenfrequency of the wind turbine.
 30. A method ofcustomizing an eigenfrequency of each of a plurality of wind turbinesin-field, comprising: for each of the plurality of wind turbines,performing the method according to claim 26.